Adjustable speed drive lifetime improvement method

ABSTRACT

The present techniques include methods and systems for operating an inverter to maintain a lifespan of the inverter. In some embodiments, the switching frequency and/or the output current of the inverter may be changed such that stress may be reduced on the inverter bond wires of the inverter. More specifically, embodiments involve calculating the aging parameters for certain operating conditions of the inverter and determining whether the operating conditions result in aging the inverter to a point which reduces the inverter lifespan below a desired lifespan. If the operating conditions reduce the inverter lifespan below the desired lifespan, the switching frequency may be reduced to a lower or minimum switching frequency of the inverter and/or the output current of the inverter may be reduced to a maximum output current at the minimum switching frequency.

BACKGROUND

The invention relates generally to the field of electrical powerconverters and inverters. More particularly, the invention relates totechniques for preventing or forestalling failure of motor drivecircuitry due to overheating.

Power inverters and converters typically employ power modules to createa desired output current waveform, which is used to power variousdevices, such as motors and other equipment. The frequency and amplitudeof the output current waveform may affect the operation of the devicesuch as by changing the speed or torque of a motor, for example. Somepower modules create the desired output current waveform through pulsewidth modulation, wherein power semiconductor switches such as insulatedgate bipolar transistors (IGBTs) are caused to switch rapidly on and offin a particular sequence so as to create an approximately sinusoidaloutput current waveform. Furthermore, high transistor switching speedstend to produce a smoother, more ideal sinusoidal waveform, which may bedesirable in some applications. For example, in heating, ventilating,and air conditioning systems a smoother sinusoidal waveform will reducesystem noise and vibrations.

Higher transistor switching speeds may tend, however, to increase thejunction temperature of the transistors, which may result in moremechanical stress and increased rates of transistor failure over time.Attempts have been made to reduce transistor failure by limiting themaximum absolute transistor junction temperatures. However, thesetechniques have failed to account for the increased stresses that tendto occur under start-up conditions or low-speed conditions, wherein thetransistors tend to experience high current at low output frequency.

It may be advantageous, therefore, to provide a system and method ofreducing IGBT thermal stress that is particularly effective understart-up conditions and low-speed, high-current conditions.Specifically, it may be advantageous to provide a method of reducingtemperature variations of the transistor junction, i.e. thesemiconductor chip itself, and the case, i.e. the package in which thesemiconductor chip is contained.

BRIEF DESCRIPTION

The present invention relates generally to a transistor protectionmechanism configuration designed to address such needs. Embodimentsinclude systems and methods of reducing the switching frequency and/oroutput current of an inverter module to avoid high junction temperaturevariation and stress on the bond wires. Embodiments also include methodsof estimating the expected junction temperature variation.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a block diagram of an exemplary application for a variablefrequency drive, in the form of a wind power converter system which mayinclude an adjustable speed inverter, in accordance with an embodimentof the present techniques;

FIG. 2 illustrates a schematic diagram of an inverter in the exemplaryapplication of FIG. 1, in accordance with an embodiment of the presenttechniques;

FIG. 3 is a plot representing the relationship between output currentand operating frequency for different switching frequencies, whilemaintaining a certain lifespan of an inverter, in accordance with anembodiment of the present techniques;

FIG. 4 is a flow chart summarizing a process for changing a switchingfrequency and/or an output current to operate an inverter such that acertain lifespan of the inverter may be maintained, in accordance withan embodiment of the present techniques;

FIG. 5 is a plot representing an operating condition which maintains orextends the lifespan of the inverter, in accordance with an embodimentof the present techniques;

FIG. 6 is a plot representing an operating condition which may have achanged switching frequency to maintain or extend the lifespan of theinverter, in accordance with an embodiment of the present techniques;and

FIG. 7 is a plot representing an operating condition which may have achanged switching frequency and/or output current to maintain a certainlifespan of the inverter, in accordance with an embodiment of thepresent techniques.

DETAILED DESCRIPTION

Embodiments of the present invention relate to reducing the mechanicalstress on solid state switching devices, such as IGBTs due to largetemperature variations of the junction. Large junction temperaturevariations may contribute to particularly high levels of mechanicalstress, because the different expansion rates of the various materialsinside the transistor package may lead to wire crack growth in wirebonds and similar contacts. Therefore, reducing junction temperaturevariations may result in a longer lasting inverter module. Inembodiments of the present invention, the junction temperature variationis controlled by controlling the switching frequency. Because thehighest junction temperature variations tend to occur during start-up orlow-speed, high-current conditions, the switching frequency may bereduced for only a short time during start-up, after which the switchingfrequency may be increased to provide a smoother sinusoidal waveform.

Turning to the figures, FIG. 1 depicts an exemplary application in theform of a wind power system 10 which may include an adjustable speedinverter, in accordance with the present disclosure. It should be notedthat a wind power system 10 is provided as one example for which presenttechniques of adjusting the speed of an inverter to forestall inverterfailure may be implemented. In various embodiments, the presenttechniques may be implemented in any electronic system (not limited to awind power system) having an inverter module.

Referring again to the example provided in FIG. 1, the wind power system10 may be suitable for capturing power from wind using turbine blades 12and converting the captured wind power into mechanical power, and themechanical power into electrical power. The system 10 may include agearbox 16 connected to the turbine rotor 14 of the turbine blades 12.The gearbox 16 may adapt the relatively low speed of the turbine rotor14 with the relatively high speed of a generator 18.

The generator 18 may convert the mechanical power into electrical power,and may be, for example, an induction generator or a synchronousgenerator. For example, the generator 18 illustrated in FIG. 1 may be adoubly fed induction generator (DFIG), which includes a rotor winding 20and a stator winding 22. The stator winding 22 of the generator 18 maybe connected to a transformer 28 which transfers electrical powerthrough inductively coupled conductors to a suitable voltage level foran electrical grid 30. The grid 30 may be an interconnected networkwhich delivers electrical power to various other electrical devices ornetworks. The rotor winding 20 of the generator 18 may be connected tothe grid 30 by the converter 24 and inverter 26 which decouplemechanical and electrical frequencies (e.g., to enable variable-speedoperation).

The system 10 may include a converter and inverter module including athree-phase AC-DC converter 24 and a three-phase DC-AC inverter 26. Theconverter 24 and inverter 26 may be linked by a DC capacitor battery 32.The converter 24 may be connected to the rotor winding 20 of thegenerator 18, and may also be referred to as the rotor side converter24. The inverter 26 may be connected to the grid 30 by the transformer28, and may also be referred to as the grid side inverter 26. Thebidirectional converter and inverter 24 and 26 may enable vector controlof the active and reactive powers delivered to the grid 30 and may alsoincrease power quality and angular stability and decrease the harmoniccontent introduced into the grid 30 (e.g., via filters).

The converter 24 and inverter 26 may be used for varying levels of powercontrol, and may sometimes output relatively high power (voltage andcurrent). The converter 24 and inverter 26 may include transistors andantiparallel diodes for switching and converting such voltages. In someembodiments, the system 10 may include one or more processors 34 forcontrolling an operation of the inverter 26. For example, and as will bediscussed, the processor 34 may change the switching frequency or outputcurrent of transistors in the inverter 26 to decrease power loss andjunction temperature variations which may be affected by the operationsof the transistors in the inverter. The processor 34 may further besuitable for executing algorithms and computing parameters associatedwith operations of the inverter.

One example of an inverter 26 in some embodiments is provided in FIG. 2.The inverter 26 may include a plurality of insulated gate bipolartransistors (IGBTs) 40 and power diodes 42, each diode 42 configuredantiparallel to a respective IGBT 40. The IGBTs 40 and power diodes 42are joined to positive or negative DC lines (as appropriate) and outputlines a, b, or c with bond wires 44. For example, the output lines a, b,and c of the output 46 may output the three-phase voltages v_(a), v_(b),and v_(c). The rapid on and off switching of the IGBTs 40 to produce adiscretized three-phase output current waveform at the output 46 mayresult in conduction losses and switching losses, which may result in ahigher junction temperature at the IGBTs 40. Such junction temperaturesmay result in strain and/or deformation of the bond wires 44, which mayshorten the lifespan of the inverter 26. Though the example of aninverter 26 provided in FIG. 2 relates generally to an inverter of awind power system 10, the present embodiments may apply to any invertermodule having switching transistors, as high junction temperatures maystrain and/or deform the transistors and/or bond wires of a typicalinverter. As used herein, an inverter 26 may refer to any invertermodule in an electronic system, and the IGBTs 40 may refer to any typeof switching transistor (and is not limited to an IGBT).

Accordingly, embodiments of the present invention include a method ofestimating the peak junction temperature in an inverter module. In someembodiments, the estimated peak junction temperature may be based on theestimated power losses of the IGBTs 40. Furthermore, the estimated powerlosses of the IGBT 40 may be based on estimated operating conditions ofthe IGBTs 40. For example, peak IGBT 40 junction temperature estimatesmay be based on estimated conduction losses and switching losses ascalculated according to the following equations:

$\begin{matrix}{{{P_{c}\left( {f,I_{RMS}} \right)} = {{\left( {\frac{1}{2 \cdot \pi} + \frac{{M(f)} \cdot {PF}}{8}} \right) \cdot V_{t} \cdot \sqrt{2} \cdot I_{RMS}} + {\left( {\frac{1}{8} + \frac{{M(f)} \cdot {PF}}{3 \cdot \pi}} \right) \cdot R_{t} \cdot 2 \cdot I_{RMS}^{2}}}},} & (1) \\{\mspace{79mu}{{{P_{s}\left( {f_{s},I_{RMS}} \right)} = {\frac{1}{\pi} \cdot f_{s} \cdot E_{onoff} \cdot \left( \frac{\sqrt{2} \cdot I_{RMS}}{I_{nom}} \right) \cdot \left( \frac{V_{DC}}{V_{nom}} \right)}},\mspace{79mu}{and}}} & (2) \\{\mspace{79mu}{{{P\left( {f,f_{s},I_{RMS}} \right)} = {{P_{c}\left( {f,I_{RMS}} \right)} + {P_{s}\left( {f_{s},I_{RMS}} \right)}}},}} & (3)\end{matrix}$where P_(c) is the estimated conduction power loss as a function of thefundamental frequency, f, and the output RMS current of the drive,I_(RMS), P_(s) is the estimated switching power losses as a function ofthe switching frequency, f_(s), and the output RMS current of the drive,I_(RMS), and P(f, f_(s), I_(RMS)) equals the total estimated powerlosses of the IGBT 40. In equation (1), M(f) represents the modulationindex and PF represents the power factor of a load driven by theinverter 26. In equation (1), V_(t) represents the approximate IGBT 40conduction voltage at small or near zero forward current and R_(t)represents the approximate slope resistance. Both V_(t) and R_(t) may bederived from a manufacturer datasheet for the transistor (e.g., IGBT 40)used in the inverter 26. In equation (2), E_(onoff) represents the totalenergy required to switch the IGBT 40 on and off at a rated voltageV_(nom) (half of the IGBT rated voltage) and current I_(nom) (rated IGBTmodule current) of the IGBT 40. All three of E_(onoff), V_(nom), andI_(nom) may be obtained from manufacturer data sheets. I_(RMS) andV_(DC) represent the estimated output current and bus voltage of theIGBT 40.

Therefore, both the output current I_(RMS) and the switching frequencyf_(s) act as scaling factors applied to the switching loss value whichcontribute to the total power loss P. The total power loss P may affectthe junction temperature variation (ΔT_(j)), which decreases thelifespan of the inverter 26. For example, the total power loss P may beused to calculate the junction temperature variation ΔT_(j) using theequations described below.

In some embodiments, the calculation of the junction temperaturevariation, ΔT_(j), may be approximated by assuming that the temperaturevariation of the case is negligible. As such, a “boost factor” (BF(f))may be first calculated according to the following equation:

$\begin{matrix}{{{{BF}(f)} = {1 + {\sum\limits_{i = 1}^{4}{\frac{R_{i}}{R_{jc}} \cdot \frac{\pi - 1}{\sqrt{1 + \left( {2{\pi \cdot f \cdot \tau_{i}}} \right)^{2}}}}}}},} & (4)\end{matrix}$where R_(i) and τ_(i) equal the thermal resistances and capacitances ofthe thermal network of the inverter 26, and R_(ic) equals the overallthermal resistance between the junction and the case. Furthermore, aninterim value, BF_ΔT_(j), may be approximated from the boost factor,according to the following equations:BF _(—) ΔT _(j)(f)=1.85·(BF(f)−1) if BF(f)<2  (5);BF _(—) ΔT _(j)(f)=BF(f) if BF(f)≧2  (6).

Having obtained the estimated power losses and the boost factor, theestimated junction temperature variation, ΔT_(j), may then beapproximated according to the following formula:ΔT _(j)(f,f _(s) ,I _(rms))=PI(f,f _(s) ,I _(rms))·BF _(—) ΔT _(j)(f)·R_(j)  (7),where ΔT_(j) represents the junction temperature variation after oneoutput cycle of the inverter module.

It will be appreciated that variations of the above formulas may be madewhile still falling within the scope of the present invention.Additionally, in some embodiments one or more of the variables, such asI_(RMS), E_(onoff) or V_(DC) for example, may be measured.Alternatively, these variables may also be estimated based on averageknown operating conditions of typical inverter modules or a particularinverter module. Additionally, in some embodiments, the diode 42junction temperature variation may be estimated rather than the IGBT 40junction temperature variation.

The mean junction temperature T_(m) can be calculated using a negativetemperature coefficient (NTC) sensor. Generally, the NTC temperaturesensor is embedded inside or on a heatsink near the IGBT module. Whenthe NTC temperature sensor is embedded near the IGBTs 40, the averagejunction temperature of the IGBTs 40 can be approximated by thefollowing equations

$\begin{matrix}{{T_{m} = {T_{ntc} + {{PI} \cdot {\sum\limits_{i = 1}^{4}\frac{R_{i}}{1 + {{sR}_{i}C_{i}}}}} + {{PI} \cdot \frac{R_{ii}}{1 + {{sR}_{ii}C_{ii}}}} + {{PI} \cdot \frac{R_{di}}{1 + {{sR}_{di}C_{di}}}}}},} & (8)\end{matrix}$where R_(i) and C_(i) represents the thermal resistance an capacitance,respectively. R_(ii) and C_(ii) represent the thermal couple resistanceand thermal couple capacitance, respectively, between IGBT power andtemperature difference between the IGBT case layer and the NTC sensor.R_(di) and C_(di) represent the thermal couple resistance andcapacitance between diode power and temperature difference between IGBTbase layer and the NTC sensor. T_(ntc) represents the temperature of thedrive measured by the NTC sensor. The parameters R_(ii), C_(ii), R_(di)and C_(di) can be extracted parameters from the inverter 26.

In some embodiments, the number of cycles to failure (N_(f)) for theIGBT can be estimated using different algorithms. For instance, oneexample of how the number of cycles to failure is estimated is providedin U.S. Patent Application Number 20090276165. In other embodiments,different methods may also be used to estimate the number of cycles tofailure, and may be based on the parameters of the inverter and/orelectronic system.

One or more embodiments include techniques for adjusting the switchingfrequency and/or output current of an IGBT 40 to possibly increase thelifespan of an inverter 26. For example, one technique may berepresented by the flow chart of the process 60 in FIG. 4. The processmay begin by calculating (block 62) the number of cycles to failure,based on the previously discussed equations. The aging per second of theinverter 26 may then be calculated (block 64) based on the calculatednumber of cycles to failure. For example, the aging speed of the drivecan be calculated by the following equation:Aging_per_second=1/N _(f)(ΔT _(j) ,T _(jmin))/f  (9).In some embodiments, the aging per second of the IGBT 40 can be furthercharacterized as a function of the switching frequency: f_(s), driveoperating frequency: f and output current I_(o).

The switching frequency (f_(s)) and/or the output current (I_(RMS)) maybe changed based on the calculated aging per second (equation (9)) ofthe IGBTs 40 in the inverter 26. In some embodiments, the processor 34,or any other suitable processor of the wind power system 10 may performalgorithms to calculate the number of cycles to failure and/or the agingper second of the inverter 26. The processor 34 may also be used tocontrol the operation (e.g., altering an IGBT 40 switching frequency oroutput current) of the inverter 26 based on the calculated results.

In some embodiments, the calculated aging per second of the inverter 26may be characterized by three scenarios, which are represented in FIGS.5, 6, and 7 as plots 78, 82, and 86, respectively. Referring again toFIG. 4, the process 60 may involve determining (block 66) whether theaging per second is below the commanded switching frequency (f_(s,cmd))of the IGBTs 40 and the maximum operating current (I_(RMS)) at thecommanded switching frequency f_(s,cmd). For example, referring to theplot 78 of FIG. 5, if the commanded switching frequency f_(s,cmd) were 4kHz, represented by the trace 52, the aging per second at the commandedparameters, represented by point 80, may not increase the aging rate ofthe inverter 26 beyond the desired lifespan of the inverter 26. Thus,referring to FIG. 4, no change may be made (block 68) to the operationof the inverter 26.

However, referring to the plot 82 of FIG. 6, if the commanded switchingfrequency f_(s,cmd) were 4 kHz, represented by the trace 52, the agingper second at the commanded parameters, represented by point 84, may beabove the maximum current 56 at the commanded switching frequency, whichmay result in increased power loss, increased junction temperaturevariation, increased inverter aging, and decreased inverter lifespan. Insome embodiments, the process 60 (FIG. 4) may involve determining (block70) whether the calculated aging per second of the commanded parametersis above the currently commanded switching frequency f_(s,cmd), butlower than a minimum switching frequency (f_(s,min)) of the inverter 26.The process may then change (block 72) the commanded switching frequencyf_(s,cmd) to a lower switching frequency. For example, the commandedswitching frequency f_(s,cmd) may be changed (block 72) to the minimumswitching frequency f_(s,min) of the inverter (e.g., 2 kHz), asrepresented by the trace 50. By setting the f_(s,cmd) to a lowerfrequency f_(s,min), the operating parameters of the point 84 may bewithin the operating range of the inverter 26 for achieving a desiredlifespan of the inverter 26.

Referring to the plot 86 of FIG. 7, if the commanded switching frequencyf_(s,cmd) were 2 kHz, represented by the trace 50, the aging per secondat the commanded parameters, represented by point 88, may be above themaximum current 56 at the commanded switching frequency. Further, as thecommanded switching frequency f_(s,cmd) may already be the minimumswitching frequency f_(s,min) of the inverter 26, the lowering theswitching frequency may not be possible for improving the aging persecond of the operating parameters. Thus, in some embodiments, both theswitching frequency f_(s) and the output current I_(RMS) may be changed.For example, the process 60 (FIG. 4) may involve determining (block 74)whether the calculated aging per second of the commanded parameters isabove the currently commanded switching frequency f_(s,cmd), and alsoabove a minimum switching frequency (f_(s,min)) of the inverter 26. Theprocess 60 may then change (block 74) the output current command I_(RMS)to the maximum output current under the minimum switching frequencyf_(s,min), as represented by the adjustment of point 88 to point 90. Insome embodiments, if the commanded switching frequency f_(s,cmd) is notalready at the minimum switching frequency f_(s,min), the commandedswitching frequency f_(s,cmd) may also be set to the minimum switchingfrequency f_(s,min). Therefore, the operating parameters of the point 90may be within the operating range of the inverter 26 for achieving adesired lifespan of the inverter 26.

In some embodiments, the process 60 may be performed dynamically orperformed at intervals. For example, the process 60 may be performed atset time intervals, or the process 60 may be performed wheneveroperating changes in the system 10 occur. By continuously applying theprocess 60, an appropriate switching frequency and/or output current maybe selected for the IGBTs 40 of the inverter 26 to maintain a desiredlifespan of the inverter 26.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

The invention claimed is:
 1. A method of operating an inverter, themethod comprising: calculating a failure parameter of the inverter;changing a commanded switching frequency of a plurality of transistorsin the inverter to a lower switching frequency if the failure parameteris above a first threshold parameter for achieving an estimated lifespanof the inverter; and changing an output current of the plurality oftransistors if the failure parameter is above the threshold parameterand if the lower switching frequency is a minimum switching frequency ofthe inverter.
 2. The method of claim 1, wherein calculating the failureparameter comprises calculating a number of cycles of the inverter to anestimated failure of the inverter.
 3. The method of claim 1, whereincalculating the failure parameter comprises calculating a junctiontemperature variation of bond wires in the inverter.
 4. The method ofclaim 1, wherein calculating the failure parameter comprises calculatinga fatigue function of the inverter.
 5. The method of claim 1, whereinthe first threshold parameter is based on the commanded switchingfrequency and a maximum output current of the inverter with respect toan operating frequency of the parameter.
 6. The method of claim 1,wherein changing the commanded switching frequency to a lower switchingfrequency results in a second threshold parameter based on the lowerswitching frequency and a maximum output current of the inverter withrespect to an operating frequency of the parameter.
 7. The method ofclaim 1, wherein the method is performed dynamically during operation ofthe inverter.
 8. The method of claim 1, wherein the method is performedat set time intervals during operation of the inverter.
 9. The method ofclaim 1, wherein the method is performed whenever operating parametersof the inverter change to result in calculating a different failureparameter.
 10. A method of delivering power to a load, the methodcomprising: switching a plurality of transistors in an inverter of anelectronic system at a command switching frequency, wherein theplurality of transistors output current at a first current level;determining a failure parameter of the plurality of transistors, basedon the command switching frequency and the first current level; reducingthe command switching frequency if the failure parameter is above athreshold; and reducing the first current level of the inverter if thefailure parameter is above the threshold and if the command switchingfrequency is approximately equal to the minimum switching frequency. 11.The method of claim 10, wherein determining the failure parametercomprises determining a junction temperature variation at the pluralityof transistors.
 12. The method of claim 10, wherein determining thefailure parameter comprises calculating an aging per second of theinverter operating at the command switching frequency and outputting atthe first current level.
 13. The method of claim 10, wherein reducingthe first current level of the inverter comprises reducing the firstcurrent level to a maximum current level of the minimum switchingfrequency.
 14. A computer readable medium comprising instructions that,in operation, is executed by a processor associated with an invertercomprising: calculating a failure parameter of the inverter; changing acommanded switching frequency of a plurality of transistors in theinverter to a lower switching frequency if the failure parameter isabove a first threshold parameter for achieving an estimated lifespan ofthe inverter; and changing an output current of the plurality oftransistors if the failure parameter is above the threshold parameterand if the lower switching frequency is at a second threshold switchingfrequency of the inverter.
 15. The computer readable medium of claim 14,comprising code for changing an output current of the plurality oftransistors if the failure parameter is above the threshold parameterand if the lower switching frequency is a minimum switching frequency ofthe inverter.
 16. A method of operating an inverter, the methodcomprising: calculating a failure parameter of the inverter; changing acommanded switching frequency of a plurality of transistors in theinverter to a different switching frequency if the failure parameter isabove a first threshold parameter for achieving an estimated lifespan ofthe inverter; and changing an output current of the plurality oftransistors if the failure parameter is above the first thresholdparameter and if the switching frequency is at a second thresholdparameter of the inverter.
 17. The method of claim 16, wherein thesecond threshold parameter is the minimum switching frequency of theinverter.
 18. The method of claim 16, wherein calculating the failureparameter comprises calculating a number of cycles of the inverter to anestimated failure of the inverter.
 19. The method of claim 16, whereincalculating the failure parameter comprises calculating a junctiontemperature variation of bond wires in the inverter.
 20. The method ofclaim 16, wherein calculating the failure parameter comprisescalculating a fatigue function of the inverter.
 21. The method of claim16, wherein the first threshold parameter is based on the commandedswitching frequency and an output current value of the inverter withrespect to an operating frequency of the parameter.
 22. The method ofclaim 16, wherein changing the commanded switching frequency to adifferent switching frequency results in a different threshold parameterbased on the different switching frequency and an output current valueof the inverter with respect to an operating frequency of the parameter.